Adverbs describe verbs and past participles, adjectives or other adverbs.

It snowed heavily.
A poorly written story.

An adverb can be one word (quickly) or a phrase (at noon).
Adverbs can describe manner (how), place (where), time (when), frequency (how often), degree (to what extent).

She dances gracefully.
He is leaving tomorrow.

Formation of adverbs

We form an adverb by adding ”ly” to the adjective: sudden - suddenly;
Adjectives ending in ”le” drop the ”e” and take ”y” - possible - possibly;
Adjectives ending in consonant + ”y” drop the ”y” and take ”ily” - easy - easily;
Adjectives ending in ”l” take ”ly” - legal - legally;
Adjectives ending in ”ic” usually take ”ally” - magic - magically;
Some adverbs do not follow these rules: good - well, fast - fast, hard - hard, late - late, early - early.